The invention relates to a method for grinding the main and rod bearings of a crankshaft by external cylindrical grinding.
Crankshafts made of steel or cast materials are mass-produced for the internal combustion engines of motor vehicles. An important factor here, in addition to economical mass production, is in particular the greatest possible accuracy with regard to diameter, roundness and centricity. Very high demands are therefore imposed on grinding methods of said type. According to EP 1 181 132 B1, it has already been recognized that the grinding result can be improved by the main and rod bearings of the crankshafts being ground in a very specific sequence.
This is because stresses are released during the grinding of the crankshafts, which to begin with are only machined by chip removal, and these stresses lead to the deformation of the crankshaft blanks during the grinding. The deformations after the grinding of the rod bearings are especially pronounced. It has therefore been proposed according to EP 1 181 132 B1 to finish-grind the rod bearings as far as possible at an early stage. The instruction is therefore given to firstly rough-grind the main bearings, then rough- and finish-grind the rod bearings and lastly finish-grind the main bearings. The known method has the advantage that deformations of the crankshaft which originate from the grinding of the rod bearings can partly be removed again during the finish grinding of the main bearings. In addition, the known method can be carried out in a single setup of the crankshaft. In this known method, the grinding started with the rough grinding of the main bearings so that the crankshaft is clamped in a precisely defined rotation axis, namely its defining geometrical longitudinal axis, for the grinding of the rod bearings. This defining geometrical longitudinal axis must be available as a reference axis for the machining of the rod bearings. On a finish-ground crankshaft, all the main bearings and also other regions of the crankshaft that are arranged concentrically to the main bearings must be oriented exactly according to the defining geometrical longitudinal axis of the crankshaft with regard to diameter, roundness, true running and centricity. The same applies to the center line of the crank journals, which again is a defining geometrical longitudinal axis for the rod bearings.
To this end, the existing geometrical longitudinal axis is established by means of centering bores at the end faces of the crankshaft. The crankshaft is clamped between centers at its centering bores and is rotationally driven by a driving device. This type of clamping has the disadvantage that a certain axial pressure has to be exerted on the crankshaft, as a result of which there is the risk of additional deformations because the crankshaft bends under the effect of an axial pressure. It is therefore necessary to also place one or more steady rests.
Attempts have also already been made to exert an axial pull on the crankshaft during the clamping of the latter. But there is still the disadvantage that additional deformations can occur during the first stage of the method according to EP 1 181 132 B1. An optimum grinding result is again made more difficult as a result; in addition, the known method thus becomes more complicated again.